CVE-2026-54340: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplific
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.
Details
Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-54340
Exploitation outlook
EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.
- Low exploitation riskCVE-2026-543400.28% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 20% of all scored CVEs.
Referenced CVEs
| CVE | CSIRTS overview | External |
|---|---|---|
| CVE-2026-54340 | coverage & exploitation status | NVD · CVE.org |
Recent advisories for h2o is an
A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.
- highCVE-2026-44453: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 6b5370d, h…nvd · 2026-07-16
- mediumCVE-2026-44452: h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, w…nvd · 2026-07-16
- highCVE-2026-44436: Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP …nvd · 2026-07-16
- highCVE-2026-44435: Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP …nvd · 2026-07-16
- mediumCVE-2026-44434: Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP …nvd · 2026-07-16
- mediumCVE-2026-44433: Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP …nvd · 2026-07-16
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